Unable to use -C of grep in Unix Shell Script - unix

Question

I am able to use grep in normal command line.
grep "ABC" Filename -C4
This is giving me the desired output which is 4 lines above and below the matched pattern line.
But if I use the same command in a Unix shell script, I am unable to grep the lines above and below the pattern. It is giving me output as the only lines where pattern is matched and an error in the end that cannot says cannot open grep : -C4
The results are similar if I use -A4 and -B4

Answer 1

I'll assume you need a portable POSIX solution without the GNU extensions (-C NUM, -A NUM, and -B NUM are all GNU, as are arguments following the pattern and/or file name).
POSIX grep can't do this, but POSIX awk can. This can be invoked as e.g. grepC -C4 "ABC" Filename (assuming it is named "grepC", is executable, and is in your $PATH):
#!/bin/sh
die() { echo "$*\nUsage: $0 [-C NUMBER] PATTERN [FILE]..." >&2; exit 2; }
CONTEXT=0 # default value
case $1 in
-C ) CONTEXT="$2"; shift 2 ;; # extract "4" from "-C 4"
-C* ) CONTEXT="${1#-C}"; shift ;; # extract "4" from "-C4"
--|-) shift ;; # no args or use std input (implicit)
-* ) [ -f "$1" ] || die "Illegal option '$1'" ;; # non-option non-file
esac
[ "$CONTEXT" -ge 0 ] 2>/dev/null || die "Invalid context '$CONTEXT'"
[ "$#" = 0 ] && die "Missing PATTERN"
PATTERN="$1"
shift
awk '
/'"$PATTERN"'/ {
match='$CONTEXT'
for(i=1; i<=CONTEXT; i++) if(NR>i) print last[i];
print
next
}
match { print; match-- }
{ for(i='$CONTEXT'; i>1; i--) last[i] = last[i-1]; last[1] = $0 }
' "$#"
This sets up die as a fatal error function, then finds the desired lines of context from your arguments (either -C NUMBER or -CNUMBER), with an error for unsupported options (unless they're files).
If the context is not a number or there is no pattern, we again fatally error out.
Otherwise, we save the pattern, shift it away, and reserve the rest of the options for handing to awk as files ("$#").
There are three stanzas in this awk call:
Match the pattern itself. This requires ending the single-quote portion of the string in order to incorporate the $PATTERN variable (which may not behave correctly if imported via awk -v). Upon that match, we store the number of lines of context into the match variable, loop through the previous lines saved in the last hash (if we've gone far enough to have had them), and print them. We then skip to the next line without evaluating the other two stanzas.
If there was a match, we need the next few lines for context. As this stanza prints them, it decrements the counter. A new match (previous stanza) will reset that count.
We need to save previous lines for recalling upon a match. This loops through the number of lines of context we care about and stores them in the last hash. The current line ($0) is stored in last[1].